Entrance insulation for electrical conductors



H. HANDREK ENTRANCE INSULATION FOR ELECTRICAL CONDU CTORS 3 Sheets-Sheet1 Filed Feb. 4, 1935 dawn/or I Nov. 23, 1937.

m W. M 0 M x W M w m o k s Q WQ Em kb w Nov. 23, 1937. H. HANDREKENTRANCE INSULATION FOR ELECTRICAL CONDUCTORS Filed Feb. 4, 1935 3Sheets-Sheet 2 W m M M w 0 w M x n m a xfimuku x Q Q MQQQN 6 xkk qkuub Li W n M w w w x n m a lumen/or Nov. 23, 1937. H. HANDREK ENTRANCEINSULATION FOR ELECTRICAL CONDUCTORS Filed Feb. 4, 1935 3 Sheets-Sheet 3Patented Nov. 23, 1937 UNITED STATES PATENT OFFICE ENTRANCE INSULATIONFOR ELECTRICAL CONDUCTORS Hans Handrek, Bad Klosterlausnitz, Germany,

assignor to Poraellaniabrik Kahla, Kahla, Germany InG

QChiml.

My invention relates to an improved entrance insulation of an electricalconductor.

The objects of my invention are a means for sealing the passage of anelectrical conductor through an insulating element of an inorganicinsulating material, more particularly through the wall of a chamber,such as an electric cell or the like, and the provision of an improvedgastight entrance insulation which may be cheaply produced and iscapable of withstanding mechanical and thermal stresses without losingits gastight character.

My invention is primarily applicable to the manufacture of radio tubesand other vacuum cells including electrodes which are connected toconductors passing through the wall of the tube made of glass orporcelain or a similar inorganic insulating material, but is alsoapplicable to structures in other arts, for instance to spark plugs.

Prior to my invention it was customary to employ conductors in the formof wire, strip, tube or rod passing through the insulating member orwall of glass, quartz or the like and sealed therein either by a fusingprocess or by packing material. Considerable difilculties wereexperienced in the fusing process due to the different coemcients ofthermal expansion of the conductor and the insulating material throughwhich the conductor passes.

For these reasons. it was not possible, for in.- stance, to produce asatisfactory entrance insulation by a fusing process of coaxiallyarranged tubular conductors of considerable diameter, although such anarrangement of the conductors is highly desirable in radio tubes forshort wave lengths.

A further object of my invention is the provision of entrance insulationof a plurality of tubular conductors arranged in nested relation passingthrough a wall of an inorganic insulating material. such as glass,quartz, porcelain and other ceramic materials used in the electricalart, more particularly, in high frequency apparatus.

I have found that the difficulties arising from the difference in thecoefficients of thermal exermany December 27, 1933 of the wall or otherelement through which the carrier member passes.

Moreover, I have found that the glaze uniting the conductive member,that is to say the carrier membenprovided with the metallic film, withthe surrounding insulating material and interposed therebetween, shouldhave a coefficient of thermal expansion from about to 40 percent lowerthan hat of the carrier member and of the surroundirlg insulatingmaterial of the wall or the like. 10

Another way of avoiding the difficulties arising from the differencebetween the coefiicients of expansion of the conductor and theinsulating element through which the latter passes, is to interposetherebetween a plurality of layers of glazes 16 having differentcoeflicients of expansion which are so chosen that the coefiicient cfthe innermost layer differs but slightly from that of the conductor,while the coefllcient of the outermost layer diiiers but slightly fromthat of the insulating material, the coefllcients of the intermediatelayers, if such are provided, lying therebetween.

Further objects of my invention will appear from the descriptionfollowing hereinafter of different embodiments illustrated in thedrawings, while the features of novelty will be pointed out in theclaims.

Fig. 1 illustrates an axial section through a tubular conductor passingthrough a wall of an an inorganic insulating material, the conductorbeing sealed therein by a plurality of layers of fused glaze;

Fig. 2 is an axial section through a high vacuum tube provided with myimproved entrance insulation of a plurality of electrode conductorsarranged in coaxial relationship;

Fig. 3 is the section taken along line 1-3 of Fig. 2;

Fig. 4 illustrates partly in section an entrance 0 insulation in which aplurality of layers of different thermal expansion are interposedbetween the conductor and the wall;

Fig. 5 is a diagram illustrating the relative coefficients of expansionof the layers shown in 5 Fig. 4;

Fig. 6 is a section through the lower portion of a high vacuum tube inwhich the conductors connected to the electrodes are provided with anentrance insulation of the type shown in Fig. 4;

Fig. '7 is a view similar to Fig. 4 of a slightly modified entranceinsulation;

Fig. 8 is a diagram showing the relative coefficients of thermalexpansion of the entrance insulation of Fig. 7;

Figs. 9 and 10 are representations similar to Figs. 7 and 8 of a furtherexample of my improved entrance insulation;

Fig. 11 illustrates in an axial section the entrance insulation of ahousing for electrical condensers;

Fig. 12. is an axial section of a conductor passing through the cover ofan electrolytic condenser and provided with my improved entranceinsulation; and

Fig. 13 is an axial section through a spark plug in which the conductorof the central electrode is provided with my improved entranceinsulation.

In Fig. l I have illustrated the insulation of the point of passage of aconductor ll through a wall it of a ceramic insulating material, forinstance the material known on the German market under the trade nameCalit".

Calit is a pure magnesium silicate free from iron, sintered at atemperature of 1800-1400 C;

It is far superior to other ceramic materials regarding its factor ofdielectric loss and is, therefore, used for high frequency purposes inpreference to other insulating materials.

The conductor II is formed by a tubular member. preferably made of thesame material as the wall H, for instance Calit, and provided with athin film ll of a conductive material. preferably metal.

Any of the well known methods may be employed for producing theconductive film H. for instance the burning process employed in theceramic art for depositing a layer of silver or gold on china were forornamental purposes. or the metal spraying process developed by Bchoopor a cathode spraying process or electrolysis. It is to be understood,of course, that while the use of a noble metal. such as silver, gold orplatinum. will facilitate the burning process if such is employed fordepositing the film, any other metal, such as copper, aluminum or thelike, may be chosen provided precautions be taken in the burning processto avoid oxidation. The burning process may be carried out, forinstance, in a reducing atmosphere free from oxygen.

Thefilm ll iswhollyorinpartcoveredbya thin layer of glaze indicated atI2. The thickness of this layer may amount to .006 inch for instance,while the conduct )1 it may have a diameter of a quarter of an inch.l'ior sealing the aperture provided in the wall ll through which theconductor It passes, I employ an enamel fiux having a coefficient ofthermal expansion which is about 10-40 per cent smaller than thecoefficient of thermal expansion of the material of which the carriermember I0 and the wall M are made.

Afterthe carrier member II has been provided with the conductive film IIand the layer of glaze II, it is fixed in a suitable refractory holderrelative to the member il in the position shown in Fig. 1. The apertureis recessed at the top' as shown in Fig. 1 and the flux is brought intothis recess in the form of a powder. The refractory holder carrying thetwoelements II and H in fixed relative position is then heated in asuitable oven to a temperature atwhich the powdered fiux will fuse andfill the gap left between the layer it and the wall of the aperture.This gap may have a width of about .006 inch.

I have found that the use of intermediate layers between the conductorand the wall of aglare or other fiux having a coeiiicient of expansion,which is higher than that of the wall or the conductor. is liable toproduce undesired thermal stresses which impair the mechanicalproperties of the insulation. Particularly. tensional stresses in theintermediate layers must be avoided and I have found that this objectmay be readily attained by the use of a glaze or iiux having a lowercoefiicient of expansion than the material of the element or wallthrough which the conductor passea; In the embodiment shown in Fig. 1for instance. I may make the layer ll of a glass having from 87 to inper cent of the coefficient of Calit. while the flux ll may preferablyhave a coefficient of heat expansion as low as 80-82 per cent of that ofCalit. A great number of glass or glase compositions are known inthe artwhich have the required characteristics.

I may use. for instance. for the layer it a silicate fiint glassreferred to in the publication tion:

Per cent A110: 5.0 8:0: 12.0 BiO: 71.95 NazO..- 11.0 llnzO: 0.05

Preferably. the flux II should have a lower melting temperature than theglaze i2 so that the glaze It will not fuse when the flux I! is molten.The composition 0 118 above referred to has a melting temperature of 800degrees Celsius, for instance, whereas the composition has a meltingpoint of 720 degrees Celsius.

If desired, the conductor ll could be directly fused into the wall It bymeans of the flux ll without being previously provided with a glaze II.The provision of the glaze I! affords the advantage of a protection ofthe conductive layer against mechanical injury. If desired, the layersIt and it may be made of the same material.

The use of conductors formed by a ceramic carrier member provided with aconductive film oflers particular advantages when applied to dischargetubes for high frequency purposes. as the electrode conductors may becoaxially arranged as illustrated in Figs. 2 and 3. Because of the wellknown skin-effect this will reduce the electrical resistance of theconductors. Owing to their elasticitythe thin conductive films depositedon a ceramic insulating carrier will not set up substantial stressestherein owing to thermal expansion. but will readily follow theexpansion and contraction of the carrier or of the surrounding materialwithout exerting any considerable stress thereon. whereas conductors inwire or strip form. such as used prior to my invention, are liablebecause of their considerable crosssection to break the fused insulatingseal, when subject to considerable temperature changes.

A multiple electrode discharge tube according to my invention is shownin Figs. 2 and 3. It com-' prises a base plate 14 preferably made ofCalit and a bulb ll, suitably fixed thereto. for instance by a fusedglaze, and consisting of glass, porcelain conductors comprise coaxiallyarranged tubes II and it of a suitable inorganic insulating material.for instance also of Calit. each provided with inner and outerconductive films II. II or ll, II respectively. which may consist of asuitable metal deposited on their carrier by the above explained burningprocess. The conductive films II, II, II and it are soldered to suitablewires leading to the different electrodes on the one hand and tosuitable terminals on the other hand. An illustration of the socket ofthe tube carrying these terminals is not deemed necessary as it does notform part of my invention.

The tube II is filled with a suitable enamel fiux ii providing for aseal. An insulating tube It preferably consisting of the same materialas the tubes II and It. for instance of Calit, is interposed between thetubes II and II in spaced relation thereto. The gap between the threenested tubes is filled by fused enamel fiuxes 22 and TI to provide for avacuum-proof seal.

The multiple electrode unit comprised of the three tubes ll, II and 2tunited with each other by the interposed layers of fiux. is finallyinserted in the aperture provided in the base plate 24 and is sealedtherein by the fusible enamel fiux It.

The coefiicients of expansion of the fiux employed for the layers 2|,2!, I1 and it are preferably 10 to 40 per cent below the coefiicient ofthermal expansion of the base 24. Instead of Calit any other ceramic orinorganic insulating material may be employed, for instance glass.quartz, quarts-glass and so on.

My invention is not limited to vacuum-proof entrance insulation, but isalso applicable to entrance insulation of conductorsextending intochambers containing gas or liquid under high pressure, or requiring atight seal from the atmosphere. Thus, my invention is applicable to theentrance insulation of the terminal conductors of electrical condensersenclosed in an insulating housing or to the entrance insulation of thecentral electrode of a spark plug.

In Fig. ii I have shown the entrance insulation of a conductor passingthrough the ceramic wall I. of a housing for electrical condensers. Thecarrier member Ii of ceramic material which may be in form 'of a rod ora tube is provided with a conductive film, for instance with a layer ofnoble metal deposited on its carrier by the burning pnocess which isordinarily employed in the ceramic art for providing china were withmetallic films for ornamental purposes. The metallic film is providedwith a protective glaze covering. The metallic film and the glaze'covering are designated by I! as a whole. The aperture provided in theceramic wall It is recessed at the top and the recess is filled with asuitable flux 83 which is fused to enter the gap left between the wallof the: aperture and the carrier member to provide for a vacuum-tightseal. The fiux I! is formed by a suitable glaze or other inorganiccompound having a coeihcient of thermal expansion amounting to but 60-90per cent of the coefficient of the material of which the'wall I. and thecarrier member ii are made. The conduc-.

tive film l2 is'made thicker at its ends as indicated at N ,and II topermit the attachment of conductingwires by soldering. v

Pig. 12-illustrates the entrance insulation of a conductor leading intoa metallic casing of an electrolytic condenser. The sheet metal cover Iof the casing is provided with an aperture accoma modating a flangedbushing. ll of a suitable insulating material, for instanceporcelain.The flange of this bushing bears upon the cover 40, a

suitable washer ll being interposed therebetween and is held in positionthereon" by a sheet metal ring I! connected to the cover it by bolts. In

order to protect the ceramic bushing ll from mechanical injury. asuitable washer ll of resilient material isintsrposed betweenthe flangeand the ring If.

A conductor 41 passes throughthe bushing I and is sealed therein inaccordance with my inv'ention. For this purpose. the conductor 41comprisesa tube or rod of a'suitable ceramic material. for instance ofporcelain provided with a metal film ll and a protective glans covering.The axal boring of the bushing II is enlarged at the top and theenlargement is filled with a molten fiux II of a suitable glase unitingthe bushing "*with the glaze covering. The coefiicient of thermal heatexpansion of the fiux it amounts preferably to 60-90 per cent of thecoefiicient of the porcelain of which the members 41 and II are made.

At the ends of the member 41 the metallic film is preferably left freefrom the protective glaze and is made thicker so that it will not bedamaged bythe attachment to the member 41 of a split ring it suitablyclamped on the member I! and attached to the wire It. A similar splitring not shown in Pig. i2 is attached-to the upper end of member 41.

Fig. 13 illustrates a spark plug in which the passage of the centralelectrode thrnugh the insulating member II is sealed inaccordance withmy invention. The spark plug comprises an outer metallic sleevedesignated by It as a whole. The lower end of this, sleeve is threadedin the customary manner as indicated at I. whereas the upper end 08 isof hexagonal cross-section to permit the engagement thereon of a wrenchand is provided with an inner thread I! permitting the insertion of anut-ring N adapted to hold the insulating member I! in place. The,member I! is provided with a shoulder tl bearing against a washerpositioned on an inner shoulder of the sleeve as. Another washer isinterposed between the ring-nut N and the insulating member ll.

Through the axial boring of the ceramic member ll, there passes a rodI'l consisting of the same ceramic material. The rod I1 is providedwitha conductive film I, for instance a metal film, and preferably witha protective glaze thereon. The axial boring of the member It isenlarged at the top'and the'space thus provided is filled with the fusedflux; as indicated at II. A metallic terminal member II is suitablyconnected to the lower end of the rod I1. Any suitable connection may beused for this purpose. Inthe embodiment illustrated inl 'ig. 13, themember II is of larger diameter than the rod I1 and is provided witha-boring into which the rod '1 is'closely fitted. .The end of the rod llextending intothe member II is left free from the protective-glue. Therod I1 is provided with a diametrical-borins registering with suitableopenings in the-member It and serving for the insertion of a rivet it. Asuitable electrode finger t1 cooperating with the member II is attachedto the sleeve It;

' The upper, end of'rod II is shown asbeing of reduced diameter and asinserted in the axial boring of abolt ll and fixed therein by atransverse-rivet it. A nut ll screwed on bolt as and bearing'on a washerll placed on the top of the ceramic body 88 serves to transmit anythrusts which may be exerted on the bolt 88 directly to the ceramic--body 88 to preserve the rod 81 from injury.

A clamping nut 12 provided on the bolt 88 permits the attachment oftheignition cable. The coeillcient of heat expansion of the flux 88 shouldbe 10-40 per cent lower than the coei'ficient of the ceramic member 88.

Sometimes, I prefer to first insert the conductor in a tube of asuitable inorganic insulating material of small diameter and to seal thesame therein by a fusible flux using a gas flame for this purpose.Subsequently, one or more of such tubes including the sealed conductorare inserted in the apertures of the wall. Suitable fiux material isthen supplied to the gaps and the whole structure is then heated in anoven whereby the flux will-unite the tubes with the wall. The wall mayconsist of the same inorganic'insulating material as the tube or tubesor may consist of metal,

as desired.

As pointed out above repeatedly, it is essenti that the coefilcient ofthermal expansion of the fiux materials should be less than that of theinorganic insulating materials by 1040 per cent. If desired, a pluralityof superimposed layers of fluxes having different coefllcients ofexpansion may be interposed between the elements to be united, so thatthe coefficient will gradually vary from the innermost to the outermostlayer. A product obtained by this process is illustrated in Fig. 4. Theconductor 84 which may be a metal wire or rod ora ceramic carrierprovided with a conductive film, for instance a metal film, passesthrough a tube 81 of Callt and is fixed therein by layers 85 and 88 of asuitable glaze. In order to reduce stresses due to thermal expansionorcontraction to a minimum, I prefer to limit the axial dimension of thelayers 88 and 88 as much as possible,

Preferably, the member 84 is first provided with the coat 85 of asuitable glaze having a relatively high melting temperature and then thecoated member 84 is inserted in the tube and is heated in a gas flamewhile the fiux 88 is applied which should melt at a temperature which isinsufiicient to iiquefy the coat 88. Glazes having such melting pointsare well known in the art and, therefore, need not be specified indetail.

After the tube 81 and the conductor 84 have thus been united, the tube81 is inserted in the aperture of the wall 88 which may also consist ofCallt. The aperture is enlarged at the top to provide for a recess whichis to be filled with a suitable flux in powder form. Thereupon the wholestructure is brought into an oven and heated therein to a temperature atwhich the flux will melt and fill the gap completely, as shown at 88.The flux filling the gap should have a coefficient of heat expansionwhich is lower than that of Calit by 10-40 per cent.

Fig. 5 shows a diagram illustrating the heat expansion properties of thedifferent materials assuming that the tube 81 and the wall 88 consist ofCallt having a coefilcient of per cent, while the conductor 84 is ofmetal having a considerably lower'coefilcient of heat expansion.

Fig. 6 illustrates this process applied to the base plate 88 of a vacuumdischarge tube for high frequency purposes, the same reference numeralsbeing inserted as in Fig. 4.

The performance of the process in two successive steps, the first stepcomprising the sealing of the conductors 84 in the tubes 81 and thesecond step the sealing of the tubes 8'- in the. wall 88, oifersparticular advantages if the wall 88 is comparatively thick or. largeand, therefore, should not be locally heated only as otherwise excessivestresses might result. It will be readily appreciated that there are noobjections to a local heating of the thin tube 81 at its upper end andthat, therefore, the conductor 84 may be easily sealed in the tube bymeans of a gas 'fiame, whereas the sealing of the various tubes 81 inthe wall 88 requires that this operation should be performed in an oven.

It is not absolutely necessary that the wall 88 to be united by thefusing process with the tube 81 enclosing the conductor 84 shouldconsist'of the same insulating material as this tube. I may make thewall 88 of some other inorganic insulating material or even of metal,although in the latter instance it is more difiicult to reliably unitethe wall with the ,tube because of the greater difference in thecoemcients of thermal expansion.

In Fig. '1 I have illustrated a thin tube at made of Calit enclosing aconductor 88 similar to conductor 84 and consisting either of a metalrod or of an insulating carrier covered by a conductivefilm. Theconductor 88 is secured and sealed in the wall 88 by the fused layers 8|and 82 of suitable composition. Similarly, the tube 88 is securedand-sealed in the wall 88 consisting of porcelain by two layers 84 and88 of a suitable compound. These compounds are so chosen that thecoefilcient of thermal expansion of the layer 84 is smaller, preferably10-40 per cent less, than the coeiiicientof Callt amounting to '1.5x10whereas the coefilcient ofthe layer 88 should be less than thecoeilicient of porcelain amountin to about 5X10-.

The selection of the compounds employed for the layers 8| and 82 iscontrolled by similar consideraitons.

In Fig. 8 I have diagrammatically indicated the relative coefilcients ofexpansion compared with that of Callt. Thus, the conductor 88 has acoemcient amounting to 69 per cent of that of Callt, layer 8| 68 percent, layers 82 and 84 84 per cent, layer 88 83 per cent and wall 88-67per cent of that of Callt.

Fig. .9 illustrates diagrammatically an example of ,a vacuum-proofelectrode conductor entrance into a metal casing. Themetallic electrodeI81 consisting .of molybdenum for instance is secured and sealed in atube I88 consisting of a ceramic material, for instance Callt, byinterposed fused layers I88 and I88. The tube I88 is inserted in asuitable aperture of the metallic wall I88 and the gap between the wallsof this aperture aaizd the tube I88 is filled by fused layers III and AFig. 10 shows atically the relative coefllcients of thermal expansion,the expansion of the metal wall I88 being given as 100 per cent. Thediagram shows that the coefiicient of anyone of he layers I8I, |82,-I88,I88 is lower than the coefilcient of the adjacent element I88, I88 orI81, preferably 10-40 per cent lower. Moreover, it will be noted thatthe coefllcient of thermal expansion of the tube I88 lies intermediatethe coefficients of the conductor I81 and of the wall I88.

The theory underlying my invention is based on the observation that athin layer of a molten flux of an inorganic insulating material, such asglass fused in contact with slightly spaced surfaces, will not withstandmechanical stresses if is the reason why the coefficient of heatexpansionof the layer should under no circumstances surpass that of theadjacent elements which are The coefncient of ceramicunited by thelayer. materials, however, will nearly always vary between certainlimits. I have found for instance, that the coefficient of thermalexpansion of Callt varies between 7.6xl0 and 7.8Xl0-'. 'Iheoretically, asatisfactory result could be attained if the coefficient of the fluxlayer would be the same as that of the ceramic element on which thelayer is fused. Because of the unavoidable variations of the coefficientof the ceramic material, however, I prefer to select a fiux materialhaving a coefiicient which is lower than that of the ceramic material.

These considerations will explain why a very slight difference less than10 per cent between the coefficients is sufiicient if the layer is fusedon a metal body. such as wall I03 or the metallic conductor I01, havinga very definite coefiicient of thermal expansion which is not subject tovariations, but may be predicted within very narrow limits. As shown inFig. 10 for instance, the coefficient of the layer I02 is but 2 per centlower than that of the metal of the wall I".

The thickness of the sealing layers is illustrated on an exaggeratedscale in all of the drawings. In practice, I have found that the gapthat is left between the wall of the aperture and the element passingtherethrough and that is to be filled with the fused flux, should have awidth of approximately .006 inch. However, the gap may be varied withinwide limits depending on the viscosity of the molten fiux. The gapshould-be so wide that the molten fiux will easily enter and yet shouldbe sufiiciently narrow to retain the molten fiux by adhesion.

Moreover, I wish it to be clearly understood that the structuresillustrated in the drawings are merely given by way of example and arenot intended to limit the scope of my invention which is capable ofnumerous modifications.

What I claim is:-

l. The combination comprising an insulating element having an aperture,a carrier of ceramic insulating material passing therethrough, aconductive film on said carrier constituting a conductor for electricalenergy passing through said element, and projecting therefrom at bothsides thereof to form electrical contact terminals, and a fiux fillingthe space between said carrier and said insulating element.

2. The combination comprising an insulating element having an aperture,a carrier of insulating material passing therethrough, a conductive filmon said carrier and a fiux filling the space between said carrier andsaid insulating element, and having a coefilciert of thermal expansionlower than that of said element or said 3. In a discharge tube, thecombination comprising a ceramic base plate having an aperture,

rial, conductive films on the inner and outer surface of said tube. andfiuxes sealing the interior of said tube and the gap between the latterand said plate.

4. In a discharge tube, the combination comprising a base plate havingan aperture, a plurality of tubes arranged'in nested relation andthrough said aperture, said plate and said tubes being of a ceramicinsulating material, conductive layers on said tubes and fluxessealing'the space within and between said tubes and said plate, thecoefiicient of thermal expansion of said fluxes being smaller than thecoefiioient of thermal expansion of said plate and said tubes.

5. The combination comprising a plate having an aperture, a'tube of aceramic material inserted in said aperture and sealed therein by a flux,

a conductor passing through said tube, and a plurality of layers ofdifferent inorganic insulating materials including glass interposedbetween said conductor and said tube and fused in intimate contact witheach other and with the conductor and with said tube.

8. The combination comprising an element having an aperture, a conductorpassing through said aperture, and a plurality of superimposed layerssealing the gap-between said conductor and said element, at least one ofsaid layers consisting of a tube of a ceramic material and the otherlayers of a fused glass, the coefiicient of thermal expansion of thelayers of glass being lower than the coefilcient of thermal expansion ofthe elements arranged in contact with said layers.

7. The combination comprising an insulating element having an aperture.a carrier of insulating material passing therethrough, a conductive filmon said carrier and a fiux filling the space between said carrier andsaid insulating element and having a coefficient of thermal expansionwhich 'is from 10 to 40 percent lower than that of said element or saidcarrier.

8. In a discharge tube, the combination comprising a ceramic base platehaving an aperture, a tube passing therethrough, said base plate andsaid tube being of an inorganic insulating material, conductive films onthe inner and outer surfaces of said tube. and fiuxes sealing theinterior of said tube and the gap between the latter and said plate, andhaving a coefiicient of thermal expansion which is from 10 to 40 percentlower than that of said tube and said plate.

9. The combination comprising a plate having an aperture, a tube of aceramic material inserted in said aperture a mass of fiux having a lowermelting point than said ceramic material and sealing said tube to saidplate, a conductor passing through said tube and a seal of glassinterposed between said conductor and said tube and fused in intimatecontact therewith, the coef ficient of thermal expansion of said fluxand of said glass being from 10% to 40% lower than that of said plateand said tube.

HANS HANDREK.

